The invention relates to an apparatus for recording and reproducing signals, particularly video signals, on a magnetic tape which is disposed in spools lying one above the other concentrically in a cassette and is adapted to be laid by a tape extracting device with a pair of grippers engaging behind the magnetic tape, in two tape loops running towards one another in opposite directions, against the periphery of a slit head drum in which at least one magnetic head rotates.
For the recording of picture signals and their reproduction on a magnetic tape, many diverse methods and apparatuses are known. The methods can be divided into longitudinal, transverse, and inclined trace recording. In longitudinal trace recording, the tape feed or speed is equal to the scanning speed. In order to achieve adequate playblack times for a predetermined length of magnetic tape the scanning speed must be kept low. However, the attainable upper frequency limit and consequently the quality of the television picture are thereby impaired. The process is therefore not suitable for small, inexpensive video recorders. In the transverse trace method the video signal is recorded in transverse tracks lying side by side. Although the tape speed is reduced in this process, nevertheless it is not possible to accommodate a television frame on a transverse track. The transverse trace method can therefore be put into practice technically only at great expense and is consequently also out of the question for inexpensive video recorders. Thus only the inclined trace method remains for such recorders.
The inclined trace or helical scan method has a number of advantages, which particularly affect the construction of inexpensive video recorders. These advantages include the low tape speed, which is between 4.75 and 19.05 centimeters per second, the good utilisation of the tape by means of inclined tracks, since an inclined track can accommodate an entire television frame, and also the relatively low electronic expense for the magnetic head and magnetic tape servo control. Finally, the good picture quality which can be achieved is also not to be ignored.
In order to achieve the necessary high relative speed between the magnetic head and the magnetic tape, the video heads rotate on a head disc inside a slit head drum around which the magnetic tape is slung helically. The video heads extend about 50 microns over the periphery of the head drum. The tape traction ensures good contact between the magnetic tape and the magnetic heads.
The speed of rotation of the head disc depends on the number of magnetic heads. In two-head systems, the head disc must rotate at 25 revolutions per second, and in single-head systems at 50 revolutions per second (in the NTSC system 30 and 60 revolutions per second respectively).
In addition to the number of video heads and the width of the magnetic tape, the angle of contact of the magnetic tape around the head drum is also of decisive importance for the functioning of the apparatus. In this respect a distinction is made between alpha-wrap covering 360.degree. and omega-wrap covering slightly more than 180.degree.. In the case of 360.degree. wrap of the head drum only one rotating magnetic head is required. The track recorded by it contains a complete television picture, so that a stationary picture can also be scanned with good picture quality. It is however extremely difficult to achieve 360.degree. wrap of the magnetic tape around the head drum. This difficulty is even greater when a video recorder has to work with cassettes. It is true that numerous tape extraction devices for cassette video recorders are already known. None of these however is able to lay the magnetic tape around the periphery of the head drum over an angle of 360.degree. or more. Techniques known from spool apparatus cannot be applied to cassette apparatus. In addition, they are usually expensive.